US2681421A - Magnetic focusing structure for electron beams - Google Patents

Description

June 15, 1954 R B, GETHMANN 2,681,421

MAGNETIC FOCUSING STRUCTURE FOR ELECTRON BEAMS Filed March 4, 1952 Fig.1.

IIII/IIIIIIIA i 50 Inventor: 5/ Richard B. Gethrhann Sb ZZMZJ D His Attorn'eg.

Patented June 15, 1954 T.MAGNETIC"TOCUSINGISTRUCTUREFOR ""ELECTRONBEAMS .sRichard :BmGethmann, Eayettevilla/N. :Y.,-. assignor: to. General Electric. "Company;v a lcorporation-of New York Ap'plicatiolFMar'ch 4, 1952-;S'erial No; 274,785

12 Claims. 1

1: @My; invention 2 relates :1 to improved; *magnetic focusing. structuressziorc cathode raywitubes and particularly to. such structures remployingipermaanent izrmagnets. as a 5 source .of :magnetomotive .Lforce.

:rInvcathode ray tubeszof ztheirtypef. presentlyiin large :9 scale; production; focusing7 of thexbeamis zaccom'plishedzby meansmof a. focusing coilz" p'osinction'ed': exteriorlm on; themeck': oi lthercathode ray Ietube. This .deviceiisxirather :bulkypexpensive and x requires-z a..1arge.:amount"-. of magnetic :material. .ISome Iattemptsthave beenwmade in thezprior. art :to accomplish .:the focusing by means of-s a -.magenetic structure supported within :the cathode :ray tube envelope. ."L'heseiz prior art structures .have .not found-commercialsuse .andahave notv rovided ;..the quality. roifocusing. necessary for usecin tubes nxfor television receivers,: for; example.

The l :present".v invention is directed to1an' im- ;proved .internalxfocusing structure; particularly a: one employingx-permanentrmagnetsewhich provides. asymmetricalifocusingfield about the axis .::of. :the focusing-istru'cture; andwhichzialso provides fora:controlled.predetermined!distribution of the magnetom'otive-fiorce along .the. axis :of. the .:focusing: structure: .;to provide f ocusing'. which is v.1substantially free; of a spherical 1aberrati'on,..-.or..:in .:many :11. cases 2-. :ivhich provides '...a predetermined 1 amount:oi.Laberration.bfithetproper .sigmto nom- 5tpensate'for:aberration-aintroducedrinto the focus- 30 *accompanyingtdrawing :zand: tszzscop s gint 'du out iniiutheSappendedtzclaims :the dra'wingiFigi 1 illustrates: an :electron; gun for; a -'rcathodelraytube.incorporating Ia: magneticiwfocus- 1;. ing.structureiembodying myunvention."

-asectionalslviewntaken .alongthedine' 2 1. 3 isiamenlargedl'elevationaliwiewzin:section or th'e' magnetid'focusing structurefiofi Figii 1.

. 2 Fig: 4:.is an elevational zviewvpartia-lly in; section 1' bf aimodifi'ed. magnetic focusing structuretem- :bodyingmyinvention, Fig. 5 isa sectional .view .utaken along the line. 5-5 of Fig. 4,; Fig; fiuis an 5 mnelevational .view. in section of a further modificationaiof.:my invention.and Fig. 7 is anxelevational nviewzin section andxpartiallybroken awaymfistill a further modification of my invention.

aReferringe now :to- Fig. 1;. Lhave :shown :my inventionziembodied yin axcathode: ray= itubezrgun rstructure" supported withinlthe. neck I not a::cath- -r.ode 'ray .tube envelope. including a zstem member .12. from::which thesgun isxsupportedand through twhich1..the.xlead;-in conductors: 3 are; sealed. .azln :1; accordance with :usual' practice. .these .aleadein conductors provide externally accessible connections to the various elements of the: gun and. also support the gun r structure within ..the s envelope. LJFDhe gunillustrated is of the :tetrode type and in-- :cludes. a. grid :member. 4 formed; asanz-zinverted x;cupihavingwarraperture. 5.;throughsawhich elec- -;trons pass. The. electrons are supplied. Joya. suit- ;zzable. .cathode 5 ,supportedvwithin the: grid: cylinder with the emitting surfaceiadjacenmthe.grid aperturerin :a' manner vwell. understood. by? those iskilledairrthe. :art. :second or accelerating; :grid ..\1 'lxof.".cuppshapeyandw having-r a *beamqaperturei 8 a. aissupported .intinverted relation with respectlto thegig'ridlcylinder lrbyzmeans of .a:::plurality :.of glass. stalks 9.secured.to:the grid cylinders Land I by means of radially. extending .pins 10.. The gun .=:strncture. also: includes a cylindrical csecond.- grid ssror anode member-I I. supported inwaxiallypspaced -.1".elation:with. respect to the accelerating rid 1 .1;by radially .-extending..pins:l2.

' The.magneticl.;focusing structure ofu thecem- ;.bodimentiillustrated Figs. Land 3;isisupported .:;direct1y :fromthe.finalpanodei cylinder .l l; which axis made: of stainless steel orsother essentially .non-

amagnetic-material. in'lihermagnetic lens-:=:member *rsiSJiIlJlZh :iorm not a machined cylinderJ 3= secured wwithin 'the .z.;anode .1 cylinder II. As..:;illustr.ated ziclearly in Fig;' 3', cylinder I l gincludes, .polepieces WI 5' :and-d 5 provided by annularend.portionswhich end portions just-ufitithe. inner."diameter.;ofi'.;;the

r finalanode tl hand which maybe. secured in posirstiongby welding to the anode. sleeve; .The iocusing a m-member! 31: is apertured as shown-ate! BFtOIDIOYldB 1-. a: passage wforw the .felectron .heam rinnwhichrwthe focusing. takes; place.- Thezrmember'll3.1-.is monstructed ima :novel mannergtazvprovide the-ade- =:..;sired:distribution a of: rmagnetomotivegforce :within. the 'ifocusing. eregionsidelined-z byzithetpassage fi-ias-willvbe:described zinidetail:at adatter point 55.-in-.the specification. 'I he iniagnetomctive force 1.2 is supplied by three cylindrical permanent magnets |1, l8 and i9 supported in circumferentially spaced relation around the exterior of the anode cylinder I and retained in position by a pair of soft iron washers and 2| which are secured against axial movement by a rolled edge 22 on the upper or outer end of the anode cylinder H and by a sleeve 23 which is spot welded to the exterior of the anode cylinder The magnets are retained in position by a pair of retaining washers 24 and 25 having suitable openings for receiving the ends of the permanent magnets and welded or otherwise secured to the opposed faces of the soft iron washers 20 and 2 The openings in the washers 20 and 2| are dimensioned to receiving the exterior of the anode H and to provide effectively a continuation of the pole pieces provided by the enlarged ends Hi and l5 of the focusing member |3. The gap in the magnetic material provided by the relatively low permeability stainless steel of the anode sleeve assists in the even distribution of a flux produced by I the three magnets and provides a circumferential tween washers 20 and 2| provided by the washer 2 la and the two air gaps.

As will be readily understood by those skilled in the artthe final anode is usually operated at the same voltage as the wall coating of the bulb and for the purpose of making electrical contact with such a coating (not shown) a plurality of circumferentially spaced spring tabs 23a are secured to the end washer 20 of the anode and focusing assembly.

.In accordance with an important aspect of the present invention, the quality of the focusing obtained is greatly improved as compared with prior art focusing arrangements. The improvement results .from a number of structural features which, though usable individually, are combined in a relatively simple structure.

In accordance with one feature of the present invention, the passage I6 through which the beam passes is defined by a continuous wall id,

as distinguished from a focusing gap. The wall is of such cross section that the magnetomotive force impressed across the ends thereof by magnets ll, l8 and I9, saturates the wall section and provides substantial focusing flux within. the passage l6. The length of the saturating section is also preferably in the order of three or more times the radius of the focusing passage. Both of these features assist in providing a focus ing field which is more uniform, and generally parallel to the axis of the focusing passage.

The term saturated as used here does not mean complete saturation with the resultant reduction 7 in permeability of the saturated section to unity but rather a saturation suflicient to reduce the permeability to a value clearly below that associated with unsaturated magnetic materials. This means that the degree of saturation may be controlled'by varying the cross section of the lens wall and in this way to obtain a non-uniform distribution of magnetomotive force. along the .tudinal' axis of the lens).

surface of the lens structure in an axial direction. In the embodiment illustrated in Fig. 3, this nonuniform thickness of the saturated wall section is obtained by machining into the member |3 a plurality of circumferential recesses 26 which extend into the member different radial distances. In the particular construction shown, the wall thickness increases toward the central portion of the lens member (measured along the longi- This particular variation in cross section of the lens tends to provide a lens having negative spherical aberration and by proper control of the amount of this aberration it can be made to just compensate for the aberration of the electrostatic lens existing in the gun structure and including, for example the lens provided by cylinders I and II which are normally operated at different voltages.

When the saturable wall section is formed in this mannen'the radial extending ribs or fins 28 provide a path for leakage flux from the magnets [1, I3 and I3 which does not afiect the focusing action of the lens and which has a relatively low reluctance circumferentially and a relatively high reluctance in a longitudinal direction. If the recesses are as deep or deeper than their axial width then the shielding accomplished by them is essentially complete and the localized leakage flux from the individual magnets does not disturb the uniformity of the focusing field.

The non-magnetic gaps interposed between the pole pieces l4 and I5 and the washers 20 and 2| respectively and provided by the anode cylinder H in Fig. 3 assist materially in obtaining a uniform distribution of magnetomotive force across the entire circumference of the pole pieces l4 and I5. This combination of the non-magnetic gaps and the shielding provided by the ribs 23 render the action of the individual magnets essentially as effective as a perfectly magnetized cylinder of permanent magnet material. This is a substantial advantage, since it is very diflicult to obtain a perfectly magnetized cylinder of permanent magnet material andin addition such a structure is rather expensive.

In Figs. 4 and 5, I have shown a modified form of my invention which is in some respects easier to assemble. The focusing member 30 is in general similar to the member |3 as shown in Fig. 3 but is difierently shaped at the ends in order to facilitate assembly with a pair of washers 3| and 32 having circular flanges 33 at the outer edges thereof and directed toward one another. The member 33 is formed with a flared portion 34 providing a shoulder on which the washer 32 may rest, and at the other end with a section 35 of reduced cross section which may be spun out over the upper surface of the washer 3| during assembly to clamp the washer 3| and 32 and the three permanent magnet members 36, 3'|.and 38 together. As illustrated in Fig. 4, a suitable non-magnetic gap between washers 3| and 32 and the parts 34 and 35 focusing member 30 is provided by interposed washers 39 and 40 of suitable non-magnetic material such as stainless steel. The washers 3| and 32 and the parts 34 and 35 of focusing member 3|] combine to provide the pole pieces of the focusing structure. As apparent from an inspection of Figs. 4 andthe flanges 33 of the washers, 3| and 32. are shaped to retain the permanent magnets in cir- .cumferential position. The assembly including the focusing. member 39, thewashers 3| 1and32 and the permanent- magnet members 36, 31 and 38 is secured on the outer end of a second anode orbrazed tothe face of the washer .32. 'be appreci'ated that the structure just described "sired. For "example, the clamped together by a stainless steel eyelet iltending circumferential flange 42 which is welded It will operates in the same manner as the one described in connection with Figs. 1, 2 and3, and

mayprovide a' more economical structure from a manufacturing point of view.

In Fig.6, I have shown a still further modi- 5 fication of myinvention in which the lens structure- 43 corresponds generallyto the lens memberl3 ofFig. 3. The lens structure instead of utilizing a saturable wall'section utilizesa stack of washers which are alternately of magnetic r an substantially non-magnetic material. Referring now to Fig; 6; the assembly includes an annular stack including soft iron p'ole pieces 44 -and-;'45 separated by a plurality of washers 46 "ofnon magnetidmaterialsuch as stainless steel with'interpcsed washers l! of high permeability iron. The washers 4t and 41 are substantially smaller outer diameter than the pole pieces 4'1 and t5 leaving room to accommodate annular cylindrical permanent magnet 48 between the pole pieces 14 and 45.

The assembly may be secured together in any suitable manner and attached to the end of the final anode, if deassembly may rbe lustrated' at 49. y

it will be noted thatin' the modification in "Big. 6 that the washers of stainless steel provide a path of relatively low permeance and the iron washers provide a path of relatively high permeance.

Since the washers of low permeability are made progressively thinner to- Ward the center of the stack and the washers of] high permeability are made progressively thicker'toward the center, it is apparent that the magnetic potential gradient is along the wall "defining the beam passage decreases toward the longitudinal center of the lens while the magnetic potential gradient is substantially constant along the axis.

This is the same type of disturbance as was obtained by the saturable wall o f the. lens of Fig. 3 where the wall section is or greater thickness near the longitudinal'center of Q the lens.

,InFig. 7, I have shown a still further modiffication to my invention which corresponds generally to the modification shown in Fig. 3 and corresponding parts have been. designated by the same reference numerals. .In the construction shown in Fig. 7, the lens member 13 includes annularpole pieces [4 and I5 and a generally cylindrical wall portion 50 extending between the pole pieces and having a varying cross section'along its length to provide the desired distributionof magnetomotive along the focusing passage. that. shown in Fig. 3 primarily in the arrangement provided for obtaining a low reluctance path. in a circumferential direction without efiec- "tively short circuiting the focusing passage in an axialor longitudinal direction. In this modi- The modification of Fig. 7 differs from fication, this path is provided by a pair of annular or ring like members 5! and 52 of low reluctance material, such as soft iron, interposed between the magnets l1, l8 and l 9 and the focusing cylinder. These rings may to advantage be supported from opposite sides of the nonmagnetic cylinder provided by the stainless steel anode cylinder 5 l which provides a non-magnetic gap therebetween. It is appreciated that the "rings- SI" and 52' are spaced" at "their ends-from the pole pieces l4. and 15 so as to provide s,

high :-r'eluctance path ina longitudinal direction. These ring members prevent the localized magnetomotiveforce of the'three magnets from disturbing the uniformity of the focusing field in .a plane transverse to the axis of the focusing cylinder.

In the-foregoing description, a number "of modifications embodying my invention have been described. It will be appreciated thatin' its broader aspects,- the invention involves the utilization of a physical structure between the pole l pieces ofa magnetic lens which {presents a relatively low' magnetic permeability as compared with unsaturated iron and which more specificallyprovidesa non-uniform and predetermined permeability along its length to give a spherical aberration free lens or a lens having a predetermined amount of special aberrationto compensate for spherical aberration of any other lens which-acts on the beam bein focused. "It has been found that for a spherical aberration freelens the area under the curve of magnetomotive force versus axial distance along the lens defining the beam passage.

"While I have shown my invention applied to a straight electron 'gun, thatisa gun having no ion trap, it will be apparent that it maybe used with such guns with all of the advantages of controlled correction ofaberration that have been described in connection with the illustrated embodiment.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A magnetic-focusing structure for an electron beam comprising i a generally cylindrical lens of magnetic material having a passage 'therethrough, said lens including a generally annular pole piece at each end thereof and. an intermediate hollow cylindrical section having a varyingcross section along its length, and meanssurrounding said lens member and impressing a magnetizing force across said pole pieces of a strength to produce saturationoi said cylindrical "section to produce a non-uniform magnetic potential gradient along the length of the inner surface of said hollow cylindrical section.

2. A'magnetic focusing structure for an electron bamcomprising a generally cylindrical lens 'memberof magnetic material having a passage therethrough, said member including a generally annular pole piece at each end thereof and an intermediate hollow cylindrical section having a varying cross section along its length to produce a non uniform magnetic potential gradient along the length" of theinner surface of said hollow cylindrical section when a magnetomotive force Orsuflicient strength to saturate said hollow "cylindrical section is impressed acrosssaid pole "pieces.

3. A magnetic focusing structure for an electron beam comprising a generally cylindrical lens member of magnetic material having a passage therethrough, said member includin a generally annular pole piece at each end thereof and an intermediate hollow cylindrical section having a cross section substantially less than the cross section of said pole pieces, and means surrounding'. said lens member and impressing a magnet izing force across said pole pieces of a strength to produce saturation of said intermediate hollow cylindrical section.

4. A magnetic focusing structure for an electron beam comprising a generally cylindrical lens member of magnetic material having a focusing passage therethrough, said member including a generally annular pole piece at each end thereof and an intermediate hollow cylindrical section, a plurality of radially extending ribs of magnetic material on the exterior of said cylindrical section and a plurality of circumferentially spaced permanent magnet members positioned outside of said cylindrical section for impressing a magnetizing force across said pole pieces.

5. A magnetic focusing structure for an electron beam comprising a generally cylindrical lens member of magnetic material having a passage therethrough, said member including a generally annular pole piece at each end thereof and an intermediate hollow cylindrical section, a ring of magnetic material surrounding each of said pole pieces, non-magnetic means interposed between the respective rings and pole pieces, a plurality of elongated permanent magnets extending between said rings and for impressing a magnetizing force across said pole pieces.

6. A magnetic focusing structure for an electron beam comprising a generally cylindrical lens member of magnetic material having a passage therethrough, said member including a generally annular pole piece at each end thereof and an intermediate hollow cylindrical section, a ring of magnetic material surrounding each ofsaidpole pieces, non-magnetic means interposed between the respective rings and pole pieces, a plurality of elongated permanent magnets extending between said rings and for impressing a inagnetizingforce across said pole pieces, said hollow cylindrical section having a plurality of longitudinally spaced outwardly extending ribs of magnetic material providing leakage paths for flux produced by said permanent magnets.

'7. A magnetic focusing structure for an electron beam comprising a generally cylindrical lens member of magnetic material having a cylindrical passage therethrough, said member including a generally annular pole piece at each end thereof and an intermediate hollow cylindrical section having a cross section substantially less than the cross section of said pole pieces, the length of said cylindrical section being in excess of three times the radius of said passage and means surrounding said lens member and impressing a magnetizing force across said pole pieces of a strength to produce saturation of said intermediate hollow cylindrical section.

8. A magnetic focusing structure for an electron beam comprising a generally cylindrical lens member of magnetic material having a cylindrical focusing passage therethrough, said member including a generally annular pole piece at each end thereof and an intermediate hollow cylindrical section having a predetermined non-uniform permeance along its length to produce a non-uniform magnetic potential gradient along the inner surface of said hollow cylindrical 'section when a magnetomotive force is impressed across said pole pieces.

9. A magnetic focusing structure for an electron beam comprising a generally cylindrical lens having a focusing passage therethrough, said lens including a generally annular pole piece of high permeance at each end thereof and an intermediate hollow cylindrical section including a stack of alternately high and low permeance rings of differing thicknesses to produce a nonuniform distribution along the length of the inner surface of said cylindrical section when a magnetomotive force is impressed across said pole pieces.

10. In combination, an electron gun for producing a beam of electrons and including a cylindrical anode, a magnetic focusing lens supported from said anode and having a cylindrical passage therethrough in alignment with the axis of said anode, said focusing lens including a pair of annular pole pieces and a relatively thin Walled generally cylindrical portion interposed between said pole pieces and means impressing a magnetomotive force across said pole pieces of a magnitude to produce substantial saturation of said wall portion.

11. A magnetic focusing structure for an electron beam comprising a focusing lens having a cylindrical passage therethrough in alignment, said focusing lens including a pair of annular pole pieces and a relatively thin walled generally cylindrical portion interposed between said pole pieces, individual magnets supported in spaced circumferential relation and said lens and impressing a magnetomotive force across said pole pieces of a magnitude to produce substantial saturation of said thin walled portion and means interposed between said magnets and said thin walled portion providing a low reluctance circumferential path around said thin walled section and a high reluctance path between said pole pieces.

12. In combination, an electron gun for producing a beam of electrons and including a cylindrical anode of non-magnetic material, a magnetic lens within said anode member including a pair of annular pole pieces having an outer surface conforming in size and shape to the interior surface of said anode, a pair of annular washers of magnetic material surrounding said anode and located opposite said pole pieces in an axial direction, and permanent magnet means positioned on the outside of said anode and extending between said washers.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,159,534 Ruska May 23,1939 2,188,579 Schlesinger Jan. 30, 1940 2,212,206 Holst et al Aug. 20, 1940 2,259,531 Miller et al Oct. 21, 1941 2,305,761 Borries et a1 Dec. 22, 1942 2,418,349 Hillier et al Apr. 1, 1947 2,503,173 Reisner Apr, 4, 1950

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